1. Field of the Invention
The present invention relates to small particles of well-defined crystalline phases, such as metals and oxides, sulfides, oxysulfides, nitrides, carbides, and the like thereof, and, more particularly, to such small particles coated with a polymerized gel to effect uniform particle size and to render desired physical and chemical properties at the particle surface. As used herein, the term "small particles" refers to particles having a diameter in the range of about 100 nm to a few micrometers.
2. Description of Related Art
Uniform particle sizes are desired in a variety of applications, such as screen-printing or layering of a phosphor-screen. Uniformity of the particles provides a higher density of packing than is obtained with particles of varying sizes.
Presently, many commercially-available powders which have a nominal particle size, actually have a rather broad distribution of particle sizes. While mechanical sieving is one technique to provide a more narrow range of particle sizes, such techniques are time-consuming, and add to the cost of the product.
Further, it is often desired to provide films of particles. However, due to the high temperatures required to sinter high melting point materials such as oxides, nitrides, borides, and carbides, cracking of the films often results.
R. K. Iler in "Inorganic Colloids for Forming Ultrastructures" and R. H. Heistand et al in "Synthesis and Processing of Submicrometer Ceramic Powders", both published in Science of Ceramic Chemical Processing, edited by L. L. Hench and D. R. Ulrich (Wiley-Interscience) 1986, disclose processes for producing particles of controlled sizes. The former reference discloses a sol-gel process for forming colloidal particles having diameters in the range of about 2 to 3 nm, considerably smaller than the particles under consideration herein. The latter reference discloses ceramic powders of controlled composition, crystallinity, size, and size distribution in a binary system, e.g., alumina-titania, achieved by reaction between the two components.
It is desired to provide powders having a substantially uniform particle size, with a narrow range of particle size distribution in a comparatively simple process. It is also desired to provide powders that may be sintered to form films, in which the sintering temperature is low enough to avoid cracking of the film.